Various display drivers exist for controlling and driving a large class of electronic display devices, such as liquid crystal displays (LCDs), light emitting diodes (LEDs), and vacuum flourescent displays (VFDs). As a family of devices, these display drivers can be configured to accommodate many different applications. For example, commercially available LED display drivers can interface to individual lamps, to seven-segment LED displays, or to combinations of both. Furthermore, display drivers are often capable of retaining data, thereby reducing control overhead. Despite design enhancements, single display drivers are often constrained by a limited number of output terminals per display driver. As a result, display drivers are commonly cascaded to expand drive capacity when driving large electronic displays with many display elements. To further increase drive capacity, display data is multiplexed onto the output terminals.
As an example of previous techniques for increasing drive capacity, consider a driver display having a ten digit, seven segments per digit, LED display. In one previous system, the LED display is driven by a single display driver which contains seventy output terminals. However, the single display driver has an inordinately high package pin count and is cost effective only in very large displays. By applying cascading techniques, the LED display can be driven in a second known system by ten separate display drivers, each driver containing seven output terminals. This second system has an added advantage of employing smaller display drivers which are cheaper to manufacture and which have increased display flexibility when driving smaller displays. By further employing multiplexing techniques in addition to the cascading technique of the second mentioned system, the LED display can be driven in a third known system by two display drivers. Each display driver contains seven output terminals which are driven in a multiplexed by five fashion. If the multiplex or display refresh rate is sufficiently fast to escape human visual detection, the ten digit LED display appears to be driven continuously. Thus, by employing multiplexing and cascading techniques, large displays with many display elements have been driven by a relatively small number of display driver devices.
For proper operation, cascaded chains of display drivers must interface with a control unit, most likely a microprocessor or microcontroller, in order to load new display data. Further, in display systems which employ cascading to increase display capacity, data display refresh rates of individual display drivers must be synchronized prior to displaying data. This synchronization process has been accomplished by circuitry either internal or external to the display drivers.
A known synchronization technique which minimizes the number of control lines and allows limited autonomy in the cascaded display driver system utilizes a synchronous serial interface. In this technique, each display driver is connected to the control unit via an enable input and a data clock signal, while communication occurs between display drivers through both data out/in signals and a synchronization signal in separate signal paths. Generally, display data is loaded into the cascaded display driver chain by the control unit during an initial configuration period, followed by independent operation of the cascaded display drivers. During independent operation, data from internal display registers are placed on the output terminals and driven onto the display array. Synchronization of individual display driver rates occurs through a dedicated synchronization signal which is fed into all display driver devices.
In the aforementioned synchronization technique, five control signals are required to implement the synchronous serial interface. In medium sized packages of approximately twenty pins, the manufacturing cost for five control signals becomes prohibitively large due to a direct relationship between pins per device and packaging costs. Continued cost reduction of display drivers has become paramount because display drivers are typically high volume, low cost devices.